Ionic Liquid Synergistic Cation-Exchange System for the Selective Extraction of Lanthanum(III) Using 2-Thenoyltrifluoroacetone and 18-Crown-6

Abstract

The mutual separation of trivalent lanthanoids is difficult due to their remarkable chemical similarity. Although the solvent extraction method is a best choice for the separation, light lanthanoids such as lanthanum (La 3+ ) generally have lower extractability than heavy ones such as lutetium (Lu 3+ ) because of their electrostatic nature or complexation ability. In other words, the selectively extractive separation of light lanthanoids from heavy ones is very difficult. Several researchers have investigated synergistic extraction systems using crown ethers including 18-crown-6 (18C6) and its analogs as size-recognizing synergistic reagents to enhance the extractability of light lanthanoids. 1 Especially, it is interesting that a synergistic extraction system using 2-thenoyltrifluoroacetone (Htta) and 18C6 as extractants and 1,2-dichloroethane having high polarity as an extraction solvent under the coexistence of hydrophobic perchlorate anion (ClO4 -) resulted in a uniquely high extractability of La 3+ as ion-pair La(tta)2(18C6)Recently, various novel extraction systems using several hydrophobic ionic liquids (ILs) as extraction phase have been investigated for the extraction of metal cations. 3,4Although neutral ligands have been mainly used in the systems including for lanthanoids, 5-7 ionic liquid chelate extraction system [8][9][10] in which an anionic (Brønsted acid-type) chelating agent such as Htta as extractant has superiority that chelates extracted into "non-volatile" IL can be back-extracted quantitatively by using an aqueous acid solution. Furthermore, in the system, metals can be extracted not only as neutral complexes but also as cationic or anionic ones by the ion-exchange process. Thus, the union of the ionic liquid chelate extraction system with the synergistic extraction system using a crown ether may result in the development of a novel selective extraction system for light lanthanoids.In this communication, we report on a fundamental study concerning a novel synergistic extraction system for a possible selective separation of light lanthanoids using an IL, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([bmim][Tf2N]), as the extraction solvent and Htta and 18C6 as extractants. As a result, a light lanthanoid, La 3+, was efficiently extracted as a cationic ternary complex by the cation-exchange process, whereas europium (Eu 3+) and Lu 3+ showed relatively low extractability without forming respective ternary complexes. ExperimentalChemicals were reagent-grade materials, and were used without further purification. Ionic liquid [bmim][Tf2N] was synthesized according to a reported procedure. 11 Distilled and deionized water was used throughout.An extraction study was performed as follows. In a centrifuge tube, an aliquot (1 cm 3 ) of [bmim][Tf2N] containing 1 ¥ 10 -2 mol dm -3 of Htta and 18C6 and 5 cm 3 of an aqueous phase containing 5 ¥ 10 -4 mol dm -3 of Ln 3+ (Ln = La, Eu or Lu), 1 ¥ 10 -1 mol dm -3 of lithium chloride and 1 ¥ 10 -2 mol dm -3 of buffer (acetic acid)...

Distribution Equilibria of Amphoteric 8-Quinolinol between 1-Alkyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)imide and Aqueous Phases and Their Effect on Ionic Liquid Chelate Extraction Behavior of Iron(III)

“…For example, several divalent transition metals 20,24 and trivalent lanthanoids 25 were extracted into ILs with 2-thenoyltrifluoroacetone (Htta) as monoanionic complexes. Moreover, the ion-exchange abilities may affect the distribution behavior of extraction reagents between the two phases.…”

Synergistic Effect of 18-Crown-6 Derivatives on Chelate Extraction of Lanthanoids(III) into an Ionic Liquid with 2-Thenoyltrifluoroacetone

“…52,53 In the use of an IL for metal cation extraction, the IL shows remarkably high performance for the extraction of alkali metals, [23][24][25][26][27] alkaline earth metals, [25][26][27][28][29][30] transition metals [31][32][33][34][35][36][37][38] and lanthanoids. [39][40][41][42][43][44][45][46] Most studies on this IL extraction system consist of the ion-exchange process with the IL cation using neutral extractants, firstly reported by Dietz and Dzielawa.…”

Highly Selective Synergism for the Extraction of Lanthanoid(III) Ions with β-Diketones and Trioctylphosphine Oxide in an Ionic Liquid

“…14,15 Recently, the present authors have reported on the synergistic ionic-liquid extraction of lanthanoids(III) (Ln(III)) with 2-thenoyltrifluoroacetone (Htta) as an acidic chelating extractant and several neutral ligands in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][Tf2N]). [16][17][18] In the Htta-18-crown-6 derivative (CE) systems, 17 a synergistic enhancement of the extractability was found for only light Ln(III) from La to Eu, leading to selective extraction of the lighter Ln, while in the Htta-trioctylphosphine oxide (TOPO) system, 18 the synergistic effect increased with an increase in the atomic number, leading to an improvement in the selectivity among Ln(III) ions. The synergism mentioned above has been ascribed to the ion-exchange extraction of specific cationic ternary complexes such as Ln(tta)2(CE) + and Ln(tta)(CE) 2+ 17 or Ln(tta)2(TOPO)3 + and Ln(tta)(TOPO)3 2+ 18 into the IL.…”